THE V.110 AND V.120 RECOMMENDATIONS FOR ISDN INTERFACES

Several of the ITU-T V Series recommendations contain specifications on how certain signals are exchanged (and changed) between different types of networks. These standards have become increasingly important in the past few years as networks, carriers, and telephone administrations have implemented the ISDN and other digital-based systems, and the ISDN standards require the use of the specifications described here. On a conventional analog-based link (see Figure 6-2), V.110 and V.I120 define how certain V. 24-interchange signals are carried on the modulated analog signal. Both user data and modem control signals are carried across the link.

They are also applicable for systems in which modem signals must be tunneled through a packet data network, such as an internet, and Figure 6-8 shows this situation.^[7] A machine called a terminal adapter (TA) is responsible for mapping the incoming V.24 signals to the appropriate bits in a packet, and sending this packet to the receiving TA.^[8] At the receiving TA, the process is reversed.

^[7] The V Series recommendations discussed in this text are offered in some vendor products, but not in all of them. If they are not available, other operations are used to perform the same types of functions.

^[8] The term terminal adapter is used by the ITU-T. In the Internet VoIP specifications, the term gateway is used. Both machines are responsible for mapping and syntax conversion operations between analog and digital systems. The TA is ISDN-oriented, and designed to use the ISDN B and D channels. The internet gateway can be ISDN-oriented, but it need not be, and it can be configured to interface with IP, SS7, and conventional telephony signals, such as onhook and offhook.

A modem modulates an analog carrier signal to convey information to another modem. This information may be user data, or it may be modem control signals. The user data is relatively straightforward: The TA maps between the analog signals and the digital bits. Control signals are more complicated. The TA must be intelligent enough to know which signals on the V.24/EIA-232 interface are to be mapped and transmitted in the packets to the receiving TA, and of course, the receiving TA must be able to perform the reverse operation.

The ITU-T defines the following specifications to perform these operations. We concentrate on V.110 and V.120 in this part of the book:

• V.100: Interconnection between public data networks (PDNs) and the public-switched telephone network

• V.110: Support by an ISDN of data terminal equipment with V Series type interfaces

• V.120: Support by an ISDN of DTEs with V Series type interfaces with provision for statistical multiplexing

• V.230: General data communications interface layer-1 specifications

V.100

The connection of user workstations to data networks through the public telephone network is quite common today. To ensure that the dial-and-answer telephone procedures are consistent across different manufacturers' equipment, the ITU-T has published V.100. This recommendation describes the procedures for physical-layer handshaking between answering and calling modems. The recommendation defines procedures for both half- and full-duplex procedures.

V.100 requires that the dial-and-answer procedures of V.25 or V.25 bis be used to perform the initial handshaking between the modems. Among other requirements, the receiving modem must send back to the transmitting modem an answer tone. Once it has transmitted this tone to the receiving modem, it enters the operations defined in V.100.

Once the modems have exchanged the dial and answer tones, the answering modem transmits what is known as the S1 signal. This signal is of a certain frequency, depending upon the type of modem used.

Upon sending signal S1, the modem remains silent until it detects a signal S2. Based on its response to S2, it either disconnects or conditions itself to the selected mode as indicated in S2. To continue the example, the originating modem sends an S1 signal to indicate it is the modem type and the receiving modem sends back an S2 signal to complete the handshake.

V.110

The V.110 recommendation has received considerable attention in the industry because it defines procedures that have been incorporated into several vendors' ISDN terminal adapters (TAs). Among other features, V.110 establishes the conventions for adapting a V Series data rate to the ISDN 64 Kbit/s rate. Figure 6-9 illustrates the scheme used by V.110.

The V.110 terminal adapter consists of two major functions: rate adapter 1 (RA1) and rate adapter 2 (RA2). The RA1 produces an intermediate rate (IR) which is then input into RA2. RA1 accepts standard V Series interface data rates, ranging from 600 bit/s to 38,400 bit/s.^[9] The k value is 0, 1, 2, or 3. The output of RA2 is always 64 kbit/s, in conformance with the ISDN B-channel rate.

^[9] The standards always lag behind the commercial industry. Obviously, 38.4 kbit/s is cited in this standard, but commercial products have higher rates.

The B-Channel Problem. It is the 64 kbit/s channelized architecture that makes the V.100-V.110 series (and ISDN) rather inefficient and inflexible with regard to the transport of VoIP traffic. We have learned VoIP is based on low bit-rate coders whose signals are not aligned on the B channel (or a DS0) boundary. Nonetheless, ISDN is a big factor on many point-to-point, dial-up links, at least for the next few years, so ISDN is a pertinent subject for this book. Anyway, let us continue the analysis.

RA Frame

Figure 6-10 illustrates the output of RA1, which is an 80-bit frame. The user data are placed into this frame, and some of the bits in the frame are also used for a variety of control functions:

• Seventeen bits are used for synchronization to provide frame alignment patterns.

  Figure 6-10. The frame structure

  

• Several bits are used to convey information about the status of V.24 circuits 105, 106, 107, 108, and 109.

• Several bits are used for network independent clocking information.

A maximum of 48 user bits can be sent in each frame. Therefore, up to 19.2 kbit/s can be placed in the intermediate frame, which has a maximum rate of 32 kbit/s. This value can be derived from a simple calculation: A 32-kbit/s channel allows 400 frames to be transmitted per second (32,000 divided by 80 equals 400). A maximum of 48 bits can be placed in each frame; therefore, 400 times 48 equals 19,200.

If a smaller data signaling rate is used, some of the positions in the frame are not relevant, and they are simply padded out with redundant data bits. For higher bit rates, RA2 creates a frame structure to handle rates of up to 64 kbit/s. For example, the user rate of 38.4 kbit/s.

We now examine how the bits in the frame are used. First, the synchronization bits are used to synchronize the machines' transmissions. The first octet serves as the initial synchronization signal and is set to all 0s. Thereafter, bit 1 of each of the following 9 octets (set to 1) completes the synchronization pattern.

The S and X bits are called status bits, and they are used to provide mapping functions of several of the V.24 interchange circuits that exist at the user device (DTE) and the TA. The state of these interchange circuits is mapped into the S and X bits, sent across the channel to the remote TA-DTE interface, and then used to operate the V.24 interchange circuits on the other side of the interface. With this approach, the system operates with digital bits in the frame, and no modems are required for the traffic inside the ISDN or packet network. The mapping scheme is as follows:

Circuit	Bit Map	Circuit
108	S1,S3,S6,S8 = SA	107
105	S4,S9 = SB	109
106	X	106

This mapping table is extracted from V.110, and can benefit from a few more explanations. The two circuit columns in the table represent the V.24 circuits at the two DTE-TA interfaces. The middle bit map column represents how the V.24 circuits are mapped into the bits in the frame. The state of the circuits (ON or OFF) are mapped to binary 0s and 1s respectively. Several of the bits are used together and are called SA and SB.

The E bits provide several functions. Some of them are used to identify the intermediate rate that is being used in the frame. Some optional E bits are used to carry network-independent clock-phase information. As an example, a modem on a public telephone network may not be synchronized to the ISDN. These bits can be used to develop phase measurements for signaling synchronization.

V.110 Handshaking

Next we discuss the operations for V.110 handshaking. Figure 6-2 is shown in two parts. Figure 6-11(a) shows the interfaces during an idle state, and Figure 6-11(b) shows the interfaces during a data transfer state. This example assumes the B channel has been established between the TAs, and they are awaiting the user DTEs to send traffic. Thus, they are in an idle state.

During the idle state, the DTEs are transmitting and receiving binary 1s on circuits 103 and 104. The TAs in turn send these 1s to each other in the B and D channels. The other pertinent circuits at the DTE-TA interfaces are ON or OFF, as depicted in the figure.

In order to send data, circuit 108/1 must be placed in the ON state, as shown in Figure 6-11(b). This change on circuit 108/1 will cause the TAs to send the frame synchronization pattern. When this pattern is recognized, then the S and X bits are sent in the ON condition. The receipt of these bits at the TAs will cause the circuits that were OFF previously to be placed in the ON condition. There are other rules (and timers) associated with these operations, but the end result is the use of circuits 103 and 104 to pass packets between the user devices.

The interface is torn down by a DTE turning to OFF circuit 108/1. The TA will then send the frame with S = OFF. The result will be turning circuits 106, 107, and 109 to the OFF condition, and the disconnection is complete.